Pressure swing adsorption (PSA) is a commonly used process for the purification of gases. Exemplary applications include separation of hydrogen from gas mixtures, separation of helium from natural gas, purification of landfill gas, and air separation for production of oxygen, nitrogen and/or argon.
Related art PSA systems are limited by their very large product and raffinate gas flow fluctuations. These fluctuations require sizeable storage or surge tanks to dampen the flow fluctuation adequately to allow proper function of downstream process equipment connected to the PSA system.
Industrial-scale gas separations have traditionally been executed using PSA cycles possessing at least one pressure-equalizing step to enhance pressurized product fractional recovery at a given purity. Increased fractional recovery decreases the amount of gas rejected to the raffinate surge tank, and ensures a more nearly continuous flow of pressurized product gas. Cycles having three or more equalizations are known.
Another step taken to reduce flow pulsation in the related art is to operate cycles having many equalizations and many vessels in a single process train. An example of a PSA system having many vessels and many equalization steps is U.S. Pat. No. 3,986,849 to Fuderer et al. which describes process trains possessing as many as ten adsorbent vessels and fifty-five valves. In industrial applications, the high energy and operating costs associated with loss of recoverable product has usually been outweighed by the considerable increase in complexity associated with more complex PSA cycles having one or more pressure equalizations except for very large plants. Thus, most plants employ extremely large surge tanks for both pressurized product and raffinate gas.
Related art PSA systems of all types, but especially those having multiple equalizations, are also subject to severe limitations due to their very high complexity and attendant high parts count. Not only does this complexity significantly increase the probability of a component failure, it also significantly increases the system size, assembly time, and material cost. Most related art PSA systems are single point of failure systems. Notable exceptions are the process revealed in U.S. Pat. No. 4,234,322 to De Meyer et al., and U.S. Pat. No. 6,699,307 by Lomax. Even in the exemplary related art processes, the PSA plant must eventually be shut-down to conduct maintenance on the defective component. Such shutdowns are extremely undesirable as they incur a significant amount of lost production time for the entire process facility. Further, when the PSA is connected to a high temperature process such as a hydrocarbon steam reformer, autothermal reformer, partial oxidation reformer, ammonia synthesis plant or ethylene cracker, the lifetime of the connected process equipment may be greatly reduced due to the high mechanical stresses incurred during a shutdown and restart event.
Keefer et al. describe systems using multiple rotary PSA modules in parallel in U.S. Pat. No. 6,051,050 in order to achieve greater overall system capacity, but do not disclose a method or strategy for operating these modules in the event of malfunction. The rotary modules of Keefer et al. are quite different than those accepted in industrial practice, and are not subject to the same type of single point valve failure as valved PSA apparatus. Their mode of failure is through gradual seal failure. The modules of Keefer also have a very large number of active beds, and they are thus less concerned with variations in product and raffinate gas flowrate pulsation. The low-pulsation rotary modules of Keefer et al., and the similar inventions of U.S. Pat. No. 5,112,367, U.S. Pat. No. 5,268,021 and U.S. Pat. No. 5,366,541 to Hill et al. suffer from inevitable leakage due to their use of sliding seals. This leakage results in reduced purity and product recovery, as well as maintenance problems due to limited seal lifetime. High pressure exacerbates these problems, making rotary modules less desirable for industrially-important separations than the valved PSA apparatuses accepted in the related art.
Because of the extremely large size of related art valved PSA systems and their very high cost, it has remained extremely undesirable to provide backup PSA capacity to prevent process shutdowns, especially for valved PSA systems having pressure equalizations and large numbers of adsorbent beds, with their attendant high complexity.
The present inventors have previously developed improved apparatuses for advanced PSA systems that greatly reduce the complexity of PSA apparatus employing pressure equalizations in U.S. Pat. No. 6,755,895 (hereafter “the '895 patent”). We have also developed new methods for executing PSA cycles which dramatically-reduce the number of valves required to execute PSA cycles in U.S. Pat. No. 6,699,307 (hereafter “the '307 patent”). We have also developed improved methods of controlling flows of purge and equalization gas in co-pending U.S. Pat. No . 6,887,301 (hereafter “the '301 patent”) as well as more advanced PSA apparatus and a novel approach to the use of multiple, modular PSA's to reduce flow variability, manufacturing cost and provide ease of service and fault tolerance in co-pending U.S. Pat. No. 6,918,953 (hereafter “the '953 patent”). All of these references are incorporated herein by reference in their entirety. Although these inventions all address the shortcomings of related art PSA's, further room for improvement remains.
The novel PSA cycles of the '307 patent are directed at separations benefiting from multiple pressure equalizations, which are generally beneficial in obtaining optimal PSA product recovery for equilibrium separations. Under certain circumstances, however, a single pressure equalization may be preferred to maximize performance. Examples include cases where an unusually-high amount of purge gas is required to strip adsorbed impurities from the adsorbent surface, where the void fraction of the adsorbent mass is low, or where the operating pressure is low. The same situation may arise when the economic value of the purified product is low and the allowable capital cost for the PSA is very low.
The modular PSA method and apparatus of the '953 patent greatly reduces the flowrate variation of a PSA plant, potentially reducing the required volume of gas storage tanks to buffer this variation, reducing the required size of piping and valving and reducing the total footprint of the PSA plant. It does, however, disadvantageously increase the number of piping connections, structural supports, etc.